Feature: Generate Code Coverage report

[lenaploetzke]

Describe your changes here: Closes #110

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General

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Tests

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[codecov[bot]]

Welcome to Codecov :tada:

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered :open_umbrella:

[holke]

I am waiting for a manual run to finish, but already give you the review comments.

[lenaploetzke]

Thank you for your review. :) I had the following problem, which is why I introduced the macro T8_CODE_COVERAGE: With test level 2 the coverage was (as expected) a bit lower than with level 0 because some code is not executed (especially some if conditions). With test level 2, however, the collection of coverage information takes too long for the ci (because we can only use 4 jobs there). Accordingly, we now use test level 1 in the ci and test a little more by using T8_CODE_COVERAGE to get the same code coverage results as with level 0. Do you maybe have a better idea to avoid the problems?

[lenaploetzke]

The corresponding Wiki article is now available under https://github.com/DLR-AMR/t8code/wiki/Generating-a-code-coverage-report. (But not yet linked in the sidebar to avoid confusion)